Geothermal and heat pump controls

How heat pump works

Heat pumps are quite different from other heating devices: contrary to them, the heat they produce is a by-product, rather than the main heat source.

Take an electric heater for example. It consumes electricity to produce heat directly. A heat pump will use this electricity to move the heat from a place to another. What’s the difference? Well it turns out that it’s cheaper to move heat than to create it. For the same amount of electricity required to produce 1kW of heat, you could move 7kW. (Of course this can vary greatly depending on a variety of factors.)

http://frozenlock.files.wordpress.com/2012/12/wpid-500px-heatpump-svg_.png?w=490

A heat pump takes heat from a place and move it to another (thus there’s always a hot side and a cold side)

If I were to put in sentimental terms, I would say that using electricity to produce heat is one of the stupidest and wasteful way to use electricity. Moving heat is far more intelligent.

Heat pump controls

Transferring heat from one place to another requires some work. The bigger the temperature difference between the two places, the more work it requires.

You could see it as moving dirt using a wheelbarrow. The temperature difference in this case would be the height difference. To take heat (wheelbarrow) from a cold place (low ground), you would have to push it to the hot place (high hill). The hotter the destination, the higher the hill.

Knowing this, to expand the least amount of energy possible, you would want to find the lowest possible hill to dump your dirt. In other words, you want your heat pump to work at a minimum temperature differential.

http://frozenlock.files.wordpress.com/2012/12/wpid-wheelbarrow_photo.jpg?w=490

To accelerate heat transfer however, you must increase the temperature difference. The heat transfer is proportional to the temperature differential squared. This means that in order to keep a room warm in the cold months of winter, you might have to increase your heater set point to a higher temperature.

Let’s sum this up in those 2 following points:

  1. For optimal efficiency, always seek the lowest temperature differential;
  2. To increase heat transfer, you have to increase the temperature differential.

Thus, we can see that the optimal way to control a heat pump is to always try to use the lowest temperature differential, and only increase it when necessary to enable a bigger heat transfer.

If we were to follow this rule and to plot it compared to the exterior temperature, this is what we would get:

http://frozenlock.files.wordpress.com/2012/12/wpid-heat-pump-optimal.png?w=490

The optimal heat pump set point should look like a mirror image of the exterior temperature

As you can see, the lower the exterior temperature, the higher the heat pump set point (to increase the heat exchange between the heat coils and the room air). But as soon as the exterior temperature rises, we lower the heat pump temperature set point.

The cold, hard, real world

Unfortunately, an optimal control is often supposed when salesman and engineers push for heat pumps, but is rarely applied once the machine is installed.

In the snowy french world of Québec, Canada, electricity is a province matter and is heavily subsidized. Even if ‘burning’ electricity directly to create heat is the dumbest way to use electricity, it’s often cheaper than to invest heavily in other solutions.

To patch this problem, the government created some ‘energy efficiency programs’. Geothermal energy and heat pumps can become a wise investment with these, but all estimates always assume an optimum heat pump control. And why shouldn’t they? If you buy a manual transmission car, everyone assumes you are going to shift gears when the time comes.

Alas, this is not the real world. Once installed, these systems are often tuned to ‘work’ and that’s it.

I’ve recently monitored a geothermal installation that reminded me of this cold hard fact. Here is what the heat pump set point looks like:

http://frozenlock.files.wordpress.com/2012/12/wpid-heat-pump-actual.png?w=490

An actual heat pump set point compared to the exterior temperature

Does this looks like a mirror image? No, it looks like a single set point for every exterior temperature. If this set point is enough for the worse case exterior temperature (-30°C perhaps?), then it’s overkill for everything else. -20°C and still the same set point? You have a needlessly big temperature differential and are thus using the heat pump in a less efficient manner.

What does it mean? Plainly, money is wasted. Remember the manual car analogy? Here the car is always in the first gear, regardless of its speed. Will the car go forward? Sure! Will you burn more fuel? Yup!

The sad part of this is that everything is in place to use it efficiently. No need to buy or replace components. Only how this system is used should be changed. This could be rectified with some lines of code… Meanwhile, electricity is wasted and money lost.

Yet another reason why everyone with a BACnet network should do everything in their power to have historical data: to spot any easy-to-change costly behaviours.


8 responses to “Geothermal and heat pump controls

  • dhaheatandcool

    Reblogged this on DHA today and commented:
    Hopefully this can help you that are thinking of a new heat pump purchase, decide if this is for you.

  • joelway

    Reblogged this on HVAC Live and commented:
    Dumbing down one of the most effective ways to heat/cool… Heat pumps!

  • Heat Pump

    Indeed a very nice post. I am also associated with Heat Pump , Air Source Heat Pump , Domestic Heat Pump . Thanks for writing such good posts and as I have subscribed to your blog, I do expect that you will be posting nice posts like this on a regular basis.

  • James Ferguson (@kWIQly)

    Superb article – right on the money – I posted a near mirror image about the stupidity of using VSD to reduce pumping costs when you could use it as feedback to reduce heat grade generated at http://blog.kwiqly.com/2011/11/eu-energy-using-products-environmental.html

    Do say hi if you like it :)

  • Edmund Resor

    Excellent, logical argument against contrary recommendations by others.

    I am a struggling homeowner trying to find a way to reduce the setpoints for water to air heat pumps and in addition to using time of day and away reductions in zone thermostats for a old (and still leaky after much work) house near White Planes, NY. The house is heat dominant and most of the time only about 67% of the square foot area is occupied. So a two stage heat pump will have ample reserve capacity.

    I cannot find good recommendations for control systems and how to actually reduce the setpoints for the hot air output. I also find a lot of cotradictory advice based presumably on the assumption the idea that the owner will use supplementary heat to recover when needed. See for example: http://www.pgov.com/GSHP/GSHP_Housing__package.pdf

    I am working with a CGD Professional Engineer to write bid documents. With $0.17/kWh for electricity, we are focusing on dual capacity HPs and considering even VFD compressors.

    Any directions to a websiite or document for a residential heat pump with adjustable, even manually adjustable, setpoints for Leaving Air Temperature would be greatly appreciated.

    Thanks for your website, and in advance, for any more help.

    Ed Resor

    • Frozenlock

      My first reflex is to think that for residential systems, a more complex control scheme might not be worth it.

      The point of the heatpump in big installation is to provide a heat/cold source from which the other systems can tap in, taking only what is required to get be able to reach their own setpoint. (Think of a heating coil in a duct, for example.)

      In a residential home, the heatpump is pretty the heat/cold source AND the final control system. There isn’t much to play with… set a comfortable temperature and enjoy the modern life with A/C ;-)

      I suppose it would be possible to install a more complex/fancy heatpump system a residential home, but I doubt it would be worth it. Additional controls, more end-points systems, more expertise… costs add up quickly and could require decades before seeing any return on investment.

      You mention that the house is leaky. This should be the primary concern. A good control is the icing on the cake. We take for granted that everything else is there. You can have the best control in the world, but it will cost a fortune to heat a house with open windows.

      Now, is it worth it to insulate even more the house? Perhaps… It really depends on what kind of energy savings you think you could get out of it.

      Best of luck to you!

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